Production of metallic iron for steelmaking is largely based on blast furnaces which produce liquid iron by chemical reduction of iron ores and melting the metallic iron. The reducing agents and the energy for sustaining the reduction reactions as well as the energy required for melting the iron is provided by combustion of coke charged to the blast furnace with air injected into said blast furnace.
Metallurgical coke is produced by pyrolysis of coal in coking furnaces. Coal is heated in order to eliminate most of the volatile components and preserving mostly the carbon structure. Coke is thereby provided with the physical and chemical properties which make it fit for providing energy and burden support in blast furnaces. Volatile matter of coal comprises a number of compounds which are distilled from coke ovens and constitute what is known as coke oven gas (COG).
The volume and composition of COG produced in coke ovens depends on the characteristics of the coal utilized. Raw coke oven gas coming from the coke oven battery has the following typical composition: water about 47%; hydrogen 29% to 55%; methane 13% to 25%; nitrogen 5% to 10%; carbon monoxide 3% to 6%; carbon dioxide 2% to 3%; hydrocarbons (ethane, propane etc.) 2% to 1%; and various contaminants such as tar vapors and light oil vapors (aromatics), consisting mainly of benzene, toluene and xylene (these three generally known as BTX); naphthalene; ammonia; hydrogen sulfide; hydrogen cyanide and other impurities.
Raw COG must be cooled, cleaned and treated in a number of chemical processes for separating valuable compounds such as ammonia and other petrochemicals and for removing sulfur, gums and other substances, so that it may be used as a fuel gas at the coke oven battery and elsewhere in the steelmaking plant. In the COG treatment plant, COG is cooled down to condense out water vapor and contaminants and for removing tar aerosols to prevent gas line/equipment fouling. Ammonia is also removed to prevent gas line corrosion, and naphthalene to prevent gas line fouling by condensation. Light oil is separated for recovery and sale of benzene, toluene and xylene, and hydrogen sulfide has to be removed to meet local emissions regulations. After this treatment, COG typically has the following composition: about 61% hydrogen; about 8% carbon monoxide; about 4% carbon dioxide; about 22% methane; about 1% nitrogen; about 2% water; about 2% of hydrocarbons heavier than methane including ethylene and acetylene; about 5% BTX; and less than about 1% of hydrogen sulfide, tars and naphthalene.
Since coke oven gas has a high calorific value, it is utilized mostly for heating purposes in steel plants, but the chemical values of hydrogen and carbon monoxide can be advantageously utilized for reduction of iron ores to metallic iron for increasing the iron/steel production of steelmaking facilities.
Direct reduction processes may be utilized in the steel industry as an alternative to blast furnaces or to supplement blast furnaces by utilizing sulfur-containing coke oven gas as a way of increasing the metallic iron production. The most common type of reactor where the DRI is produced is a shaft-type moving-bed furnace, having two main sections: a reduction zone where a reducing gas is circulated at a high temperature and through which said reducing gas is recycled in a reduction circuit and a cooling zone located below the reduction zone where the DRI is cooled down to ambient temperatures before being discharged from said reactor by circulating a cooling gas containing also hydrogen and carbon monoxide in a cooling circuit.
Iron-containing particles in the form of pellets, lumps or mixtures thereof are charged to the upper part of a shaft-type reduction reactor and are reduced to metallic iron by contacting said particles with a reducing gas containing hydrogen and carbon monoxide at temperatures above 850° C.
Oxygen is removed from the iron ores by chemical reactions based on hydrogen (H2) and carbon monoxide (CO), for the production of Direct Reduced Iron (DRI) having a high degree of metallization (ratio of metallic iron to total iron content in the DRI). The overall reduction reactions involved in the process are well known and are represented below:Fe2O3+3H2→2Fe+3H2O  (1)Fe2O3+3CO→2Fe+3CO2  (2)
The hydrogen and carbon monoxide transformed into water and carbon dioxide according to reactions (1) and (2) are separated from the gas stream circulating in the reduction circuit and are substituted by a make-up feed of reducing gas. The reducing gas make-up generally comes from a natural gas reformer, but according to the invention, this make-up gas is COG. The DRI present in the cooling/discharge zone contributes in removing heavy hydrocarbons, BTX, tars and other undesirable compounds present in the COG, whereby these substances are not present in the reduction circuit and fouling problems in the gas heater and other equipment are avoided.
There have been several proposals for utilizing COG in direct reduction processes, for example U.S. Pat. No. 4,253,867 discloses a method of using COG for reducing iron ores wherein a mixture of COG and steam is fed to an intermediate zone located between the reduction zone and the cooling zone of the reduction reactor. Coke oven gas is reformed to hydrogen and carbon monoxide in the reforming zone taking advantage of the catalytic action of the iron and the high temperature of the solid DRI in said reforming zone. This patent does not teach nor suggests any solution for solving the carbon deposition problems when the coke oven gas is heated before being fed to the reduction reactor.
U.S. Pat. No. 4,270,739 and U.S. Pat. No. 4,351,513 disclose a direct reduction process where a sulfur-containing gas such as coke oven gas is desulfurized by the iron-containing particles contained in the reduction reactor by heating and injecting the COG above the reduction zone of the reduction furnace. In the '739 patent, COG is heated in a fired heater before its introduction to the desulfurizing zone; and in the '513 patent, COG is heated by heat-exchange with the flue gases of a reformer. These patents do not even visualize the problems that arise when the COG is heated and forms carbon deposits in the heating equipment therefore no proposal for solving this problem is found in these patents.
U.S. Pat. Nos. 3,365,387, 3,557,241, 3,641,190, and others disclose some proposed methods of cleaning (decoking) process heater tubes wherein hydrocarbon-containing fluids are heated and therefore some carbon deposits are formed within the fluid heating path. The teaching of these patents is that the carbon deposits can be eliminated by reaction with steam and/or air and may be done while the heater is in operation by isolating a heating tube by means of valves for subjecting said pipe to cleaning and the rest of tubes continue with its normal operation, or shutting down the heater and subjecting all tubes to the carbon cleaning process.
None of the above patents however teaches or suggests a special design or arrangement of a heater associated to a direct reduction plant for efficiently heating coke oven gas which presents special problems because of its content of BTX and other complex carbon compounds.
No other relevant prior art has been found concerning the heating of coke oven gas for its use in direct reduction processes.
The prior art proposals present at least one of the following main disadvantages: High oxygen consumption in the case of partial combustion of COG, clogging and fouling of heater tubes, or operation limitations in the case of utilizing the lower portion of the direct reduction reactor for destroying the BTX. The COG is mixed with the recycled process gas, and finally fed to the process gas heater, only after being partially combusted with oxygen in order to completely destroy components that can polymerize and/or give cracking and generate fouling also at low temperature. This configuration requires a high amount of oxygen with the further disadvantage that almost all the methane included in the COG is oxidized and then not available for carbon deposition in the DRI. The partial combustion has additionally a negative effect on the amount of the available reducing agents. If the COG is fed directly to the Process Gas (PG) heater together with the recycled process gas, due to the high content of contaminants, the fouling will clog the heater tubes. If the COG is fed as cooling gas into the cooling/discharge reactor zone in order to promote the BTX and TAR removal, there can be use of up-flow of the COG into the reduction zone as make up of reducing agents. However, the temperatures reached in the cooling section are not enough for complete BTX removal with the consequent presence of these contaminants in the water used for cooling gas quenching. Furthermore, control of the carbon content in the DRI is affected and high carbon DRI only is produced. Also, the DRI discharge temperature is relatively low and therefore the DRI cannot be hot briquetted or directly fed into an electric arc furnace.
Documents cited in this text (including the patents discussed herein), and all documents cited or referenced in the documents cited in this text, are incorporated herein by reference. Documents incorporated by reference into this text or any teachings therein may be used in the practice of this invention.